Electrical measuring instruments



May 23, 1961 Filed May 7, 1958 J. T. WINTERMUTE ET AL ELECTRICALMEASURING INSTRUMENTS 2 Sheets-Sheet 1 Fig.2.

WITNESSES:

INVENTORS Joseph T Wintermute and ATTOR N EY y 23, 1961 J. T. WINTERMUTEET AL 2,985,832

ELECTRICAL MEASURING INSTRUMENTS Filed May 7, 1958 2 Sheets-Sheet 2United States Patent 2,985,832 ELECTRICAL MEASURING INSTRUMENTS- JosephT. Wintermute, Mountainside, and Henry Otzmann, J12, Roselle Park, N Jassignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation of Pennsylvania Filed May 7, 1958, Ser. No. 733,513 9Ciaims. (Cl. 324-151) This invention relates to electrical instrumentsand has particular relation to direct-current measuring instruments ofthe DArsonval type. Such instruments include a moving coil assemblywhich is mounted for movement through the field of a permanent magnetassembly. When the coil assembly is energized a torque results whichmoves the coil assembly relative to the permanent magnet assembly.

A number of previous constructions of various forms have been utilized.In one known construction, a permanent magnet in the form of acylindrical core has been employed which is spaced from and surroundedby an annular magnetic return path. A coil assembly is positioned with acoil side extending through the air gap defined by the space between thepermanent magnet and surrounding return path.

Another arrangement provides a two-piece permanent magnet assembly ofannular configuration which surrounds a non-permanent cylindricalmagnetic core in spaced concentric relation therewith. This constructionis represented by the Faus Patent No. 1,985,082.

According to the present invention, a direct-current instrument isprovided which has a magnetic field arrangement of improvedconstruction. In the present invention, a permanent magnet of integralone-piece construction having a toroidal configuration is provided whichsurrounds a non-permanent magnetic core of cylindrical configuration inspaced concentric relation to provide an air gap arcuate about the axisof the permanent magnet. The magnetic circuit is completed by theprovision of a magnetic band which surrounds the magnet in engagementwith the outer lateral surface thereof. The magnet is magnetizedradially of its axis to produce magnetic flux which traverses the airgap, the core and the band in directions transverse to the axis.

In a preferred embodiment of the invention, the magnet is formed of amolded mass consisting of a suitable binding material having finelydivided parts of a high coercive permanent magnet material agglutinatedtherein. This mass is pressed into the desired configuration and thenpermanently magnetized in any suitable manner. The magnet isconveniently molded into the surrounding magnetic band.

In order to provide an efiicient magnetic field assembly, the permanentmagnet is formed with diametrically opposed sections of reducedcross-sectional area taken in a plane which includes the axis of themagnet. These reduced sections provide integral connections for twohalves of the magnet. This construction is effective to minimize theshunting of magnetic flux away from the air gap between adjacentportions of the two magnet halves which contain magnetic poles ofopposite polarity.

The invention further provide a direct-current instrument havingimproved mounting means for mounting the operating parts of theinstrument. An integral one-piece supporting member carries the magneticcore and is supported by the permanent magnet to constitute the solesupport for the core. This member possesses spaced slots to permitassembly and disassembly of the coil with respect to the core when thesupporting member is detached from the permanent magnet.

The invention further provides that the magnetic field assembly ismounted by a pair of spaced post means having parts located beyondopposing end surfaces of the field assembly. These extending parts arespaced by bridges which serve to connect the extending post parts. Thepost parts are preferably of hollow construction to receive suitablesecuring means to mount the field assembly rigidly in its operatingposition.

The connecting bridges conveniently serve to mount the coil assembly inthe spaces between the bridges and between the post means for rotationwith respect to the magnetic field assembly. A suitable indicatingpointer is secured to the coil assembly to traverse a suitablecalibrated scale plate. The arrangement is such that the pointer ismovable through an angle of approximately about the axis of rotation.

In a further aspect of the invention, suitable pointer stop elements areloosely mounted at opposing ends of the path of movement of the pointerin such path of movement to prevent damage to the pointer when thepointer is energized to move in an off scale direction. Conveniently,the pointer stops are mounted by one of the connecting bridges fordisplacement with respect to the field assembly in response toengagement thereof by the pointer.

The pointer stops also serve to facilitate the mounting of the scaleplate by engaging spaced recesses of the scale plate when this plate isproperly positioned for mounting with respect to the pointer. The scaleplate is conveniently mounted by the connecting bridge which supportsthe pointer stops.

A connector device is also provided according to the invention forconnecting the coil assembly to input terminals of the instrument eitherdirectly or through suitable impedance means, depending upon theparticular application of the instrument. The connector is preferably inthe form of an electroconductive strip having one end electricallyconnected to the coil assembly. The strip is mounted for rotationrelative to the coil assembly between two spaced positions. In one ofthese positions, the free end of the strip engages one of the inputterminals and in the other position such free end is spaced from theinput terminal. At this spaced position, the strip may be connected tosuitable impedance means which is connected to the input terminal.

This connector also conveniently serves to mount a coil adjusting devicein operative position. For this purpose, the end of the strip which iconnected to the coil assembly is in the form of a spring washer and islocated to surround a bearing screw for the coil assembly on which thecoil adjuster is also mounted.

It is, therefore, an object of the present invention to provide anelectrical measuring instrument of improved construction.

It is another object of the invention to provide an improved magneticfield assembly for a direct-current instrument.

It is a further object of the invention to provide a direct-currentinstrument including a radially magnetized integral permanent magnet oftoroidal configuration surrounding a non-permanent magnetic core inspaced concentric relation to define an air gap arcuate about the axisof the magnet for receiving a moving coil assembly.

It is still another object of the invention to provide a direct-currentinstrument having a magnetic field system supported by a pair of spacedparallel post means with spaced bridges connecting parts of the postmeans which are located beyond the end faces of the field system forsupporting a coil in the spaces between the post means and connectingbridges for rotation with respect to the field system.

It is a still further object of the invention to provide adirect-current instrument having a non-permanent magnetic coresurrounded by an annular permanent magnetic means with an integralsupport member secured to the permanent magnet means to constitute thesole support for the magnetic core.

It is another object of the invention to provide an instrument asdefined in the preceding paragraph wherein the integral support isconfigured to permit ready installation and removal of a coil assemblywith respect to the core when the support is detached from the permanentmagnet means.

It is another object of the invention to provide an electrical measuringinstrument having a magnetic field system with a coil and pointerassembly mounted for rotation relative to the system and pointer stopmeans mounted in the path of movement of the pointer for displacementrelative to the system when the stop means is engaged by the pointer.

It is a further object of the invention to provide an electricalinstrument having a moving coil assembly and input terminals for theinstrument with a connector electrically connected to the coil assemblyand movable with respect to the coil assembly between a first positionwherein the connector engages an input terminal and a second positionwherein the connector is spaced from the input terminal.

Other objects of the invention will become apparent from the followingdescription taken in connection with the accompanying drawings, inwhich:

Figure 1 is a plan view of an electrical instrument constructed inaccordance with the invention with parts broken away and with partsremoved;

Fig. 2 is a view in side elevation of the instrument of Fig. l, withparts shown in section;

Fig. 3 is a plan view of the magnetic field assembly of the instrumentof Fig. 1;

Fig. 4 is a plan view of a portion of the field assembly of Fig. 3; and

Fig. 5 is a view in side elevation of another portion of the fieldassembly of Fig. 3.

Referring to the drawings, there is illustrated in Fig. 1 an electricalmeasuring instrument represented generally by the numeral 1. Theinstrument 1 is illustrated in the form of a direct-current instrumentof the DArsonval type, including a moving coil assembly which is movablethrough the field of a magnetic field assembly.

The instrument 1 includes a supporting base 3 preferably constructed ofa suitable electrical insulating and non-magnetic material forsupporting the operating parts of the instrument. A suitable coverassembly, not shown, may be removably associated with the base 3 toprovide an enclosure for the parts of the instrument.

The instrument 1 further includes a magnetic field assembly 5 consistingof a permanent magnet 7 for establishing a magnetic field through whicha coil assembly described hereinafter is movable. According to thepresent invention, the magnet 7 is of integral one-piece constructionhaving a toroidal configuration which is rectangular in cross section asbest shown in Figs. 2 and 4. The magnet 7 includes an outer lateralsurface 9 and an inner lateral surface 11 defining a central opening 13of the magnet as shown in Fig. 4. The magnet is permanently magnetizedradially of its axis so that magnetic flux flows in directions extendingtransverse to the axis of the magnet. Further details of construction ofthe magnet 7 will be described hereinafter.

In order to provide an eflicient magnetic field assembly, the inventionprovides a cylindrical core 15 which is preferably formed of asuitablenon-permanent magnetic material, such as soft iron. As best shown inFig. 3, the core 15 is located in the opening 13 of the magnet in spacedconcentric relation with the magnet to define with the inner surface 11an air gap 16 which is arcuate about the axis of concentricity.

In order to complete the magnetic circuit of the field assembly 5, theinvention provides a band or ring 17 formed of a suitable non-permanentmagnetic material which surrounds the magnet 7 in engagement with theouter surface 9 thereof. It is noted with reference to Fig. 2 that theband 17 has end portions which project beyond the opposing end faces ofthe magnet 7. With the construction as described, magnetic flux from themagnet 7 flows through the air gap, the core and the magnetic band indirections extending transverse to the axis of the field assembly.

The permanent magnet 7 is constructed according to the invention bymolding a mass consisting of a binding material having finely dividedparticles of permanent magnetic material agglutinated therein. Anysuitable binding material may be employed. For example, the bindingmaterial may comprise a phenol formaldehyde resin. The permanentmagnetic material may be in the form of finely divided particles of analloy of nickel, aluminum, iron and cobalt known as Alnico having highcoercivity and a high remanence. The powdered permanent magnetic alloyis thoroughly mixed with the binding material and this mixture is thenmolded or otherwise formed into a solid mass having the desiredconfiguration and dimensions. The molding may be accompanied by thesimultaneous application of heat to the mass if desired. Subsequent tothe molding operation, the mass is permanently magnetized to provide thepermanent magnet.

The mixture of the magnetic particles and the binding medium may bemolded into the magnetic band 17 to provide a very rigid connectionbetween the magnet and band having precise dimensions without thenecessity of finish machining, such as grinding of the magnet.

The magnet is subjected to a magnetic field which magnetizes the magnetmaterial in the manner illustrated in Fig. 4. As there shown, the magnetis magnetized radially of its axis such that magnetic poles of onepolarity are located adjacent the inner surface 11 of one side of aplanewhich includes the axis of the magnet, and magnetic poles of theopposite polarity are located adjacent the surface 11 on the other sideof this plane.

. With the arrangement illustrated in Figs. 3 and 4, the

magnetic circuit for magnetic flux from the magnet may be traced fromthe north magnetic poles of the lower section of the magnet as viewed inFig. 4, through the lower portion of the air gap as viewed in Fig. 3,the magnetic core 15, the upper portion of the air gap as viewed in Fig.3, the upper section of the magnet as viewed'in Fig. 4, and through thereturn band 17 back to the north magnetic poles of the lower section, asviewed in Fig. 4. In order to provide an efficient magnetic fieldassembly with substantially uniform distribution of magnetic fluxthroughout the air gap, the invention provides that the magnet 7 isformed with a pair of webs each of reduced cross-sectional area taken ina plane which includes the axis of the magnet for connecting two halvesof the magnet. These webs exhibit'substantial magnetic reluctance andminimize the shunting of magnetic flux away from the air gap through thewebs between adjacent portions of the two magnetic halves havingmagnetic poles of opposite polarity.

As shown in Fig. 4, the magnet 7 is formed with a pair of diametricallyopposed recesses 19 and 21 which extend radially of the magnet axis andwhich open at the end faces of the magnet. The width dimensions of theserecesses are made suificiently large so as to minimize the amount ofmagnetic flux flowing between the north and south magnetic poles of thetwo magnet halves which are adjacent the recesses 19 and 21.

I For the purpose of further minimizing the shunted magnetic flux and toalso permit supporting of the field assembly in the instrument,additional recesses 23" and are provided in the present invention whichcommuni ca-te with the recesses 19 and 21. The recesses 23 and 25 extendradially of the magnet axis toward the outer surface 9 of the magnet andopen at the end faces of the magnet. It is observed that the widthdimensions of the recesses 23 and 25 are smaller than the correspondingdimensions of the recesses 19 and 21. This arrangement provides astepped formation of the surface 11 of the magnet, as viewed in Fig. 4.The recesses 23 and 25 form webs 27 and 29 of the magnet having reducedcross-sectional areas which constitute integral connections for twohalves 31 and 33 of the magnet. These connections possess substantialmagnetic reluctance, thereby minimizing the amount of magnetic fluxwhich flows between adjacent poles of opposite polarity in the magnethalves 31 and 33. As a result, substantially the entire amount ofmagnetic flux produced by the magnet flows through the air gap portionsand the magnetic core 15.

The webs also serve to increase the rigidity and accuracy of the magnetin addition to increasing the magnetic flux which is available. Also,the webs permit a more accurate and effective bond to the surroundingmagnetic band 17.

The present invention also provides improved supporting means forsupporting the operating parts of the instrument 1. In the presentinvention, a nonmagnetic support member of integral one-piececonstruction is carried by the magnet 7 for mounting the core 15 in itsproper position. As illustrated in Figs. 3 and 5, a fiat plate 35 havinga generally disc-shaped configuration carries the core 15 which may besecured to the disc 35 in any suitable manner, such as by riveting. Theplate 35 includes an interrupted peripheral rim portion 36 with anintegral bridge 37 connecting diametrically opposed portions of the rim.The core 15 is secured to the enlarged central part 39 of the bridge 37to be concentrically disposed with respect to the rim 36 in spacedrelation thereto. The central part 39 of the bridge is connected to thespaced rim portions by two necks 41 and 43, as best shown in Fig. 3.

As illustrated in Figs. 2 and 3, the plate 35 is proportioned to besnugly received within the area bounded by the protruding portions ofthe band 17 to engage the upper end face of the magnet 7, as viewed inFig. 2.

In order to permit installation and removal of an associated coilassembly with respect to the core 15, the plate 35 is provided with apair of spaced slots which extend radially of the plate through the rimportion 36. These slots are proportioned to receive spaced parallel coilsides of the coil assembly to permit positioning of the coil assembly insurrounding relation with the core 15 as will be described hereinafter.Assembly and disassembly of the coil is efiected when the plate 35 isdetached from the magnetic field assembly. In order to permit attachmentof the plate to the field assembly, the rim section 36 is provided witha pair of diametrically spaced openings 49 and 51 which are positionedto be aligned with the recesses 23 and 25 when the plate is properlypositioned with respect to the magnet 7.

In order to permit mounting the magnetic field system 5, the base 3 isprovided with a pair of spaced posts 53 and 55 which extend upwardlyfrom the base, as viewed in Fig. 2. These posts contain respectivelyopenings 57 and 59 which extend entirely through the base 3 as seen inFig. 2. The posts 53 and 55 are proportioned to be received in the areaformed by the lower end face of the magnet 7 and the protruding endparts of the band 17 to engage the magnet end face, as illustrated inFig. 2.

For the purpose of assisting in the mounting of parts of the instrument,there is provided an upper non-magnetic electroconductive bracket asviewed in Fig. 2 of integral one-piece construction having asubstantially U-shaped configuration. This bracket is represented by thenumeral 61 and includes a central bridge section 63 which connects apair of spaced parallel posts or legs 65 and 67. The posts 65 and 67includes respectively openings 69 and 71 which extend entirelytherethrough. The bracket 61 is positioned such that the posts 65 and 67engage the upper face of the plate 35 with the bridge section 63 spacedfrom the magnet end face, as clearly shown in Fig. 2. When the bracketis so positioned, the openings 69 and 71 are aligned with the recesses23 and 25 of the magnet.

In order to secure the bracket 61 and the magnetic field assembly 5 tothe base 3, suitable securing means may be passed through the two setsof aligned openings and recesses. In the embodiment illustrated, thesecuring means may comprise non-magnetic studs 73 and 75 having enlargedheads and threaded end portions. These studs are passed through the setsof aligned openings through cavities 8t) and 81 of the base 3, such thatthe threaded end portions extend through the openings of the bracket 61.Suitable threaded nuts 77 and 79 threadedly engage the end portions ofthe studs to rigidly secure the bracket 61 and the magnetic fieldassembly to the base 3. The cavities 80 and 81 are formed withrectangular cross-sectional areas to hold captive the similarly shapedenlarged head portions of the studs.

The moving system of the instrument 1 comprises a coil assembly 82illustrated in Fig. 2 as including a frame 33 of rectangularconfiguration which is preferably formed of an electroconductivematerial, such as aluminum. The frame 83 has wound thereupon a number ofturns of electroconductive wire 85 to provide a pair of spaced parallelcoil sides of the assembly 82 which are proportioned to extend throughdiametrically opposed parts of the air gap 16 parallel to the axis ofthe core 15.

The coil assembly 82 is conveniently installed with respect to the core15 to surround the core prior to assembly of the plate 35 and core 15 tothe field assembly 5. To this end, each of the coil sides 87 and 89 ispositioned in a separate one of the slots 45 and 47 of the plate 35 toextend parallel to the axis of the rim 36. The coil assembly is thenmoved in a direction radially of the axis of the rim through the slotsto a position wherein the coil assembly surrounds the core 15. Thereverse procedure is eifected to disassemble the coil assembly from theplate and core device.

in order to mount the coil assembly 82 for rotation with respect to thefield assembly, a pair of spaced up er and lower bearing assemblies 91and 93 are provided. The upper assembly 91 includes a bearing screw 95which is threadedly received in a threaded opening of the centralsection 63 of the bracket 61. The lower assembly 93 similarly includes abearing screw 97 which is threadedly contained in a threaded opening ofa bridge section 99 of the base 3 which connects the posts 53 and 55.Suitable pivots are secured to the coil assembly in a. line parallel toand intermediate of the coil sides 87 and 89 to engage the bearingscrews 95 and 97.

In order to bias the moving coil assembly toward its zero indicatingposition, a pair of spiral bias springs and 107 are provided. Each ofthese springs has its inner end secured to the coil assembly and itsouter end secured to a portion of an adjustable member carried by thesup port means of the instrument.

These adjustable members are provided for the purpose of adjusting thebias exerted by the bias springs to thereby adjust the position of thecoil assembly. This permits a setting of the associated indicatingpointer to a zero position when the instrument is deenergized. One ofthe adjustable members 199 is in the form of a plate having an openingthrough which the screw 95 extends and a slot 111 as shown in Fig. l.The member 109 includes further a depending leg 113 illustrated in Fig.2, which is secured as by soldering to the outer end of the bias spring105. The slot 111 receives a projection of the cover, not shown. Thisprojection is rotatable from a position externally of the cover toeffect the adjustment.

A similar adjustment is provided by the adjuster 115 which is in theform of a disc having a central opening through which the bearing screw97 extends. The adjuster 115 is provided with a depending leg 116 whichis secured to the outer end of the bias spring 107. The adjuster 115 ismounted for rotation about the axis of the bearing screws to eiiectadjustment of the coil assembly by means of a threaded nut 117 whichthreadedly engages the bearing screw 97 and by means of a spring washer119. The adjuster 115 is positioned between the nut 117 and the washer119, such that the nut bears against the adjuster to force the springwasher into biased engage- .ment with the bridge 99 of the base 3.

As illustrated in Fig. l, a suitable indicating pointer 121 is securedto the coil assembly 82, such that the pointer is included in a planewhich also includes each of the coil sides 87 and 89. With thearrangement illustrated in Fig. l, the coil assembly and pointer arerotatable about the axis of the permanent magnet through an angle ofapproximately 100 about the axis of rotation.

In order to prevent damage to the pointer in response to movementthereof beyond its defined range, the invention provides a pair ofspaced stop elements 123 and 125 which are loosely mounted by thesupporting structure in the path of movement of the pointer 121. Asillustrated in Fig. l, the stops 123 and 125 are in the form ofcylindrical pins which reside loosely in pockets 127 and 129 formed inextensions of the posts 65 and 67 of the bracket 61. The pins andpockets are proportioned such that engagement of the pins by the pointerresults in displacement of the pins in the direction of pointer travel,thereby minimizing the likelihood of damage to the pointer resultingfrom such engagement. The pins are also displaced about their axes inresponse to engagement thereof by the pointer so as to present differentsurface portions to the pointer for each engagement.

The pointer 121 traverses a scale plate 131 which is located beneath thepointer 9 as viewed in Fig. 1 and which contains a suitably calibratedscale 133. The scale plate 131 includes a pair of spaced recesses, onlyone of which is shown in Fig. 1 represented by the numeral 135. Theserecesses are located in the plate 131 so as to receive the stops 123 and125 when the scale plate is in its proper position with respect to thesupporting structure and pointer. The scale plate includes further apair of openings, only one of which is shown, which is represented bythe numeral 137. These openings are positioned to receive suitablesecuring screws which extend through openings formed in projections ofthe posts 65 and 67 of the bracket 61. When the recesses 135 are inengagement with the stops 123 and 125, the openings 137 of the scaleplate will be aligned with the corresponding openings of the bracket 61to permit passage of the securing screws therethrough.

In order to permit energization of the moving coil assembly, a pair ofspaced electroconductive input terminals 139 and 141 are provided whichare conveniently carried by the base 3. These terminals extend entirelythrough the base and have portions located on opposing sides of thebase. For the purpose of connecting the moving coil assembly to theterminals 139 and 141, electroconductive connecting means are provided.As shown in Fig. 1, an electroconductive flexible strip 143 has one endwhich is connected to one of the terminals such as the terminal 141,with the other end connected to the bracket 61 by suitable securingmeans which pass through an opening 145 of the strip 143 and an openingof the bracket 61.

An additional connector is provided to connect the remaining inputterminal to the coil assembly. For this purpose, a flexibleelectroconductive strip 147 is provided which has an end electricallyconnected to the coil assembly.' In accordance with the invention, theend of the strip 147 which is connected to the coil assembly also servesas the spring Washer 119. This strip end has an opening through whichthe bearing screw 97 extends. As viewed in Fig. 2, the spring end alsoincludes a plurality of spaced depending resilient feet 148 which arebiased into engagement with the base 3 through action of the nut 117.

The invention further provides that the strip 147 is rotatable about anaxis between a pair of spaced positions to permit connection of the coilassembly either directly to one of the input terminals or throughsuitable impedance means, depending upon the particular application ofthe instrument. For this purpose, the strip 147 may be rotated about theaxis of the bearing screws to move the'free end 149 thereof between theterminal 139 and an insulating projection 155 of the base 3 which islocated on the same radius as the terminal 139 from'the axis of thebearings. t t

The free end 149 of the strip 147 has an opening 157 for receivingeither the terminal 139 or the projection 155, depending upon theposition of movement of the strip 147. When the strip is positioned toengage the projection 155, it may be conected to a terminal of suitableimpedance means, the other terminal of which may be connected toadditional impedance means or to the terminal 139.

The coil assembly is energized by current which traverses a pathincluding the terminals 139 and 141, the strips 143 and 147, a portionof the bracket 61, the adjusters 109 and 115, the springs and 107 andthe coil assembly.

With the arrangement described the distribution of markings on the scale133 is substantially uniform. For certain purposes it may be desired tomodify such uniform distribution to provide a non-uniform scaledistribution. This may be accomplished in a number of ways. For example,the core 15 may be shaped to provide an air gap of non-uniform length tothereby change the flux density distribution compared to that obtainedin a uniform gap. If desired, the magnet'7 may be nonuniforrnlymagnetized and be employed with a uniform air gap to obtain a particularnon-uniform scale distribution. i

Although the invention has been described with reference to certainspecific embodiments thereof, numerous modifications are possible and itis desired to cover all modifications falling within the spirit andscope of the invention.

We claim as our invention:

1. A magnetic assembly comprising a toroidal body of integralconstruction formed of permanent magnetic material and having inner andouter peripheral walls and end walls through which extends the axis ofgeneration of said toroidal body, said body being magnetized indirectionsextending radially of said axis such that first magnetic polesof one polarity are located adjacent said body inner wall on one side ofa plane containing said axis, and second magnetic poles of the oppositepolarity are located adjacent said inner wall on the other side of saidplane; said inner wall having a pair of diametrically opposed radiallyextending first recesses opening through said end walls, each of saidfirst recesses communicating with a separate radially extending secondrecess, each of said second recesses opening through said end walls toreceive support means for said body, said recesses cooperating with saidbody to provide a pair of spaced restricted cross-sectional areaportions extending transversely throughsaid plane, and a non-permanentmagnetic ring surrounding said body in intimate engagement with saidbody outer wall, said ring having a higher permeability than thematerial of said body.

2. In an electrical instrument, a magnetic assembly including a pair ofspaced concentric magnetic members defining an air gap arcuate about theaxis of concentricity, one of said members being permanently'magnetizedto produce magnetic flux which traverses the air gap and the other ofsaid members in directions radially of said axis, a pair of spaced postmeans extending along lines substantially parallel to said axis whichintersect the magnetic assembly, each of said post means engaging themagnetic assembly and having sections positioned beyond the ends of themagnetic assembly, a pair of spaced bridge portions each extendingbetween the pair of post sections at a separate end of the magneticassembly, a pair of spaced bearing means each carried by a separate oneof said bridge portions, a coil assembly including a coil side, saidbearing means mounting said coil assembly in the spaces between saidbridge portions and said post means with said coil side extendingparallel to said axis through the air gap for rotation about said axis,an indicating pointer carried by said coil assembly, said pointer being.included in a plane which also includes said axis and said coil side,spaced stop means mounted by said post means in the path of movement ofsaid pointer, one of said bridge portions having apertures looselyreceiving said stop means for movement relative to said post means, eachsaid stop means having an outwardly facing shoulder and a flate scaleplate carried by said one bridge portion in a plane transverse to saidaxis to cooperate with said pointer, said scale plate including spacedrecesses positioned to individually receive said stop means when saidplate is in its proper position for mounting, said plate beingengageable with said shoulders to hold said stop means in saidapertures.

3. In an electrical instrument, a cylindrical magnetic core, a permanentmagnet assembly comprising a toroidal body having inner and outerperipheral surfaces and having end surfaces through which the axis ofgeneration of said body extends, said body being of molded integralconstruction and being formed of a binding medium with finely dividedmagnetic material agglutinated in said medium, said core being of lesserdiameter than said inner peripheral surface and located within saidmagnet in spaced concentric relation therewith to provide a cylindricalconstant radius air gap, a non-permanent magnetic band surrounding saidbody in intimate engagement with said outer peripheral surface, saidbody being molded directly to said band, said inner peripheral surfacehaving diametrically opposed recesses each opening through said endsurfaces, said recesses providing reduced sections of said body whichform integral connections of increased magnetic reluctance which dividesaid body into two halves, said body being magnetized radially of saidaxis to have magnetic poles of one polarity adjacent said innerperipheral surface of one half of said body and to have magnetic polesof the opposite polarity adjacent said inner peripheral surface of theother half of said body, a coil assembly including a pair of spaced coilsides, and means mounting said coil sides in said air gap for rotationabout said axis, said reduced sections and said recesses beingproportioned to minimize the magnetic flux which is shunted from the airgap through adjacent connected portions of said magnet halves.

4. In an electrical instrument, an insulating base, a magnetic assembly,a coil assembly having two ends, means including a bearing screw carriedby said base for supporting said coil assembly for rotation about theaxis of said screw, a plate having a central opening through which saidscrew extends, a spiral bias spring connected between one end of saidcoil assembly and said plate, a put in threaded engagement with saidscrew to engage one side of said plate, an electro-conductive striphaving a pair of spaced ends one of which has an opening and a pluralityof spaced resilient depending feet, said screw extending through saidopening with said one strip end in engagement with the other side of theplate to have said feet in biased engagement with said base, a pair ofinput terminals carried by said base, said strip being rotatable aboutsaid axis to permit movement of the other end of said strip betweenfirst and second spaced positions, said other strip end engaging one ofsaid terminals at said f rst position, and being spaced from said oneterminal at said second position, and means connecting the other of saidterminals to the other end of said coil.

5. In an electrical instrument, a first toroidal member of magneticmaterial and having concentric inner and outer peripheral walls andspaced end walls joining said peripheral walls, a cylindrical member ofmagnetic material and having an outer cylindrical wall and spaced endWalls, one of said materials being permanent mag netic material, thedistance between said end walls of said toroidal member beingsubstantially equal to the distance between said end walls of saidcylindrical member, a ring member of non-permanent magnetic materialhaving an inner wall surrounding and in intimate engagement with saidouter peripheral wall of said toroidal member, said ring member havingguide portions extending beyond one of said end walls of said toroidalmember, a C-sha-ped plate-like supporting member of non-magneticmaterial having a first surface thereof seating against said one endwall of said toroidal member and having an outer wall portion withspaced portions engaging said guide portions of said ring member toaccurately locate said supporting member relative to said toroidalmember, said supporting member having an integral bridge portionoverlying the center of curvature of said toroidal member,

' means securing one end wall of said cylindrical member to said bridgeportion whereby said bridge portion holds said cylindrical memberconcentric with said inner peripheral wall, said last named means beingthe sole support for said cylindrical member, and means securing saidsupporting member to said one end wall of said toroidal member.

6. In an electrical instrument, an insulating base, a magnetic assembly,a coil assembly having two ends, means including a bearing screw carriedby said base for supporting said coil assembly for rotation about theaxis of said screw, a plate having a central opening through which saidscrew extends, a spiral bias spring connected between one end of saidcoil assembly and said plate, a nut in threaded engagement with saidscrew to engage one side of said plate, an electroconductive striphaving a pair of spaced ends, means resiliently securing one end portionof said strip to said base and including said screw whereby said stripmay be rotatively adjusted about said screw, a pair of input terminalscarried by said base, said strip being rotatable about said axis topermit movement of the other end of said strip between first and secondspaced positions, said other strip end engaging one of said terminals atsaid first position and being spaced from said one terminal at saidsecond position, and means connecting the other of said terminals to theother end of said coil.

7. In an electrical instrument, a base member having an outwardly facingsurface, a pair of spaced studs of nonmagnetic material carried by saidbase and extending outwardly of said surface, a magnetic assemblycomprising a high permeability ring and a toroidal permanent magnethaving spaced radially extending end surfaces positioned within saidring, said toroidal magnet having an inner peripheral surface and anouter peripheral surface, said outer surface seating against said ring,said magnet having a pair of diametrically spaced apertures openinthrough said inner peripheral surface and outwardly of said endsurfaces, said magnet assembly being positioned with one of said endsurfaces against said base member surface, said studs extending throughsaid apertures and outwardly of the other of said end surfaces of saidbase member, said studs being cooperable with the walls of saidapertures to locate said magnet assembly on said base member surface,said ring having locating portions extending axially outwardly of saidother base member end surface, a non-magnetic disc member having aninterrupted peripheral rim including a rim opening and a diametricallyextending integral bridge extending from an intermediate portion of saidrim into said rim opening, said disc member having peripheral portionsseating against 11 7 said locating portions of said ring, a cylindricalmember of magnetic material carried by said disc member and locatedconcentrically within said toroidal magnet, said cylindrical memberhaving an outer peripheral surface spaced from said inner peripheralsurface of said toroidal magnet to provide an annular air gap, a coilassembly including a pair of spaced coil sides, and means mounting saidcoil sides in said air gap for movement through said gap and carried atleast in part by said studs.

8. A magnetic assembly comprising a toroidal body of integralconstruction formed of permanent magnetic material and having inner andouter peripheral walls and end walls through which extends the axis ofgeneration of said toroidal body, said body being magnetized indirections extending radially of said axis such that first magneticpoles of one polarity are located adjacent said body inner wall on oneside of a plane containing said axis and second magnetic poles of theopposite polarity are located adjacent said inner wall on the other sideof said plane, said inner wall having a pair of diametrically opposedradially extending recesses opening through said end Walls, saidrecesses cooperating with said body to provide a pair of spacedrestricted cross-sectional area portions extending transversely throughsaid plane, and a non-permanent magnetic ring surrounding said body inintimate engagement with said body outer wall, said ring having a higherpermeability than the material of said body.

9. A magnetic assembly comprising a toroidal body of integralconstruction formed of permanent magnetic material and having inner andouter peripheral walls and end walls through which extends the axis ofgeneration of said toroidal body, said body being magnetized indirections extending radially of said axis such that first magneticpoles of one polarity are located adjacent said body inner Wall on oneside of a plane containing said axis and second magnetic poles of theopposite polarity are located adjacent said inner Wall on the other'sideof said plane, said inner wall having a pair of diametrically opposedradially extending recesses opening through said end walls, saidrecesses cooperating with said body to provide a pair of spacedrestricted cross-sectional area portions extending transversely throughsaid plane, and a non-permanent magnetic ring surrounding said body inintimate engagement with said body outer wall, said ring having ahigher'permeability than the material of said body, said ring extendingbeyond each of said end Walls of said body.

References (Cited in the file of this patent UNITED STATES PATENTS1,597,256 Weston Aug. 24, 1926 1,927,346 Lawrence Sept. 19, 19331,985,082 Faus Dec. 18, 1934 2,110,680 Rowell Mar. 8, 1938 2,394,113Seaver Feb. 5, 1946 2,430,317 Wilson Nov. 4, 1947 2,607,812 Lederer Aug.19, 1952 2,704,349 Sheehan Mar. 15, 1955 2,724,174 Mendelsohn Nov. 22,1955 2,837,716 Wolferz June 3, 1958 FOREIGN PATENTS 563,442 GreatBritain Aug. 15, 1944 OTHER REFERENCES Publication, Permanent MagnetManual No. 2, by the Indiana Steel Products Co., Valparaiso, Indiana,Sept. 16, 1943. Pages 16 and 23 are relied on.

